Method of making ductile superconductors



April 5, 1966 E. J. SAUR METHOD OF MAKING DUCTILE SUPERCONDUOTORS FiledAug. 12, 1963 WIND INTO Fig.8

INVENTOR. EU GEN J. SAUR United States Patent Ofiice 3,243,871 PatentedApr. 5, 1966 3,243,871 NLETHOD F AKING DUCTILE SUPERCONDUCTORS Eugen J.Saur, Giessen, Germanyg assignor to National Research Corporation,Cambridge, Mass., a corporation of Massachusetts Filed, Aug. 12, 1963,Ser. No. 301,494

Claims. (Cl. 29-1555) The invention is applicable to hard superconductoralloys comprising, as a higher melting point component,

indium.

For a fuller understanding of the nature and objects of the Invention,reference should be had to the following detailed description inconjunction with the accompanying FIG. 1 is a schematic view of thefirst step of the process of the invention;

FIG. 2 is a cross-section of the product of the invention at an earlystage of treatment; 1

FIG. 3 is a cross-section of the product at a later stage 5' 7 oftreatment; 1

FIG. 4 is a schematic view of a still further treatment; FIG. 5 is across section of'the product after the treatment indicated in FIG. 4,the section being typical of any cross-section along the length of theproduct except for the ends thereof;

FIGS. 6 and 7 are typical cross-sections of the product after furthertreatments;

I FIG. 8 is a block diagram of final treatments; and

FIG. 9 shows the resultant product in cross-section.

(2) Pressing into a disk.

(3) Spinning the disk into a tube (several passes with vacuum annealingafter each pass).

(4) Vacuum casting tin into the tube.

(5) Furnace cooling.

(6) Rolling and drawing the tube down to wire.

Conventional metallurgical processes are utilized in all the steps.Steps 1 and 4 and the anneals of Step 3 are carried out m a vacuumfurnace. The temperature of the niobium tube and molten tin in Step 4should be maintained at about 300 C., after initially annealing theniobium at higher temperature, with the vacuum furnace method of Allen,Das andStaufler, disclosed in the copending United States applications,S.N. 193,281 filed May 8,1962 and S.N. 278,723, filed May 7, 1963. ThisThe ingot 20 is pressed into the form of a disk as shown in FIG. 2. Thedisk is spun into the form of a tube 24, shown in FIG. 3, having a inchouter diameter and .040 inch wall thickness. The tube 24 is thus of highpurity and free of seams or other join The niobium tube 24 is returnedto the vacuum chamber 10. where it is annealed under a vacuum of 10- mm.V C. and 1400 C. for one hour. The chamber is provided-with a crucible 6and crucible pivots 28. Tin is melted in the crucible at 300 C. andunder a vacuum of 10- mm. Hg. Then :he crucible is tilted to vacuum castthe tin into tube 24. Tube 24 is cooled and then removed from thefurnace. The resultant product is a cylinder 32 comprising a niobiumsheath 24 and a solid tin core 30, as shown in FIG. 5. Cylinder 32 iscapped by crimping the open end. Then cylinder 32 is rolled and drawndown to wire size to form the wire 34 of FIG. 6. The rolling and drawingis carried out in accord with the teachings of Allen, Das and Stautferin their US. patent application, S.N. 193,281, filed May 8, 1962. Thediameter of the resultant wire 34 is on the order of .010 to .020 inchafter a reduction of cross-section area in excess of 30:1. The extensivereduction creates new surface at the niobium-tin interface, thusdispersing the residual contaminants therein to improve the cleanlinessof the interface.

Wire 34 is flattened between pressure rolls to form the ribbon 36 ofFIG. 7. Ribbon 36 is -then drawn to form a reduced thickness ribbonwhich is less than about .002 inch thick. The ribbon comprises anunbroken, sea-mless sheath of niobium surrounding a flattened core Theribbon is then fabricated into a solenoid according to the stepsoutlined in FIG. 8. The heating is carried out between 900 C. and 1200C. for /z to two hours. The clean-niobium surface is readily wettable bymolten tin and the reaction proceeds rapidly and uniformly. At the endof the heating cycle, the ribbon is removed from the furnace and cooledin air. The ribbon may then be insulated by a conventional plasticwhereas the insulation of prior art core wires would be a special hightemperature ceramic insulation because the prior art cores must beinsulated before heat treatment.

FIG. 9 shows the ribbon at the end of the heat treatment step. Thesheath 24 then comprises a diffusion layer 40 of Nb Sn at the innersurface of the sheath. The core 30 comprises residual tin and small voidspaces. Essentially, no niobium or Nb Sn particlesare found in the core,apart from the niobium and Nb Sn trapped in the sheaths inner diffusionlayer.

The present invention is particularly applicable to vanadium galliumwhich can only be formed with great difiiculty in prior art core wiresand is a very brittle compound. In applying the technique of the presentinvention, the tube 24 of FIG. 4 (which would be vanadium) should becapped by inserting a stopper in the open end of the tube after pouringthe gallium and indenting the tube into depressions in the stopper. Thiswill hold the low melting gallium during the drawing process. Theprocess of the present invention guards the highly reactive gallium fromcontamination.

A principal advantage of the present invention is that the ribbon isheat treated before insulating and forming into a coil. It is notnecessary to apply an expensive ceramic insulation as in prior art corewires. A cheap plastic insulation can be used. A related advantage isthatthe reactive tin or gallium or indium, etc. is trapped inside theribbon. The ribbon can be wound into a compact spiral, inserted into afurnace muflie, and then heat treated and readily unwound after heattreatment. A ribbon with the low melting component on an outer surfacewould pose problems of adjacent turns sticking if wound into a tightspiral and heated.

The above-described preferred embodiment has several advantages over theprior art. The flattened form of the final product permits a higherratio of length of Nb Sn diffusion layer to total cross-section areathan a round wire because the geometry of a ribbon is inherently morefavorable and because ribbons can be cold worked to smaller thicknessesmore readily than wires. The ribbon form inhibits the formation of voidsduring the heat treatment and is a more favorable shape than round wirefor winding coils. The wire 34, comprising a soft core 30 of tin and nohard materials, is readily flattened.

The process permits the use of high purity sheath material and vacuumdegassed core material. The clean core material is degassed while heldin crucible 26 and while being transferred from vacuum degassingcrucible 26 to tube 24. The tin is then trapped in the tube withoutexposure to oxidizing agents and other contaminants which would limitthe effectiveness of the subsequent heat treatment.

Other advantages are afforded by maintaining a continuous sheath of pureniobium about the core throughout the cold work and heating steps. Theleakage of tin is prevented and the niobium-tin interface is protectedfrom contaminants.

In comparison to the known core wires of the prior art,

the present invention offers the further advantage of confining thesuperconductive alloy to a thin diffusion layer which permits bending ofthe finished product and entails shorter times of heat treatmentcompared to the '20 hour heat treatments needed for prior art corewires.

Since certain changes can be made in the above product and processwithout departing from the scope of the invention herein involved, it isintended that all matter contained in the above description shall beinterpreted as illustrative and not in a limiting sense.

What is claimed is:

1. A method of making ductile superconductors providing high currentdensities through an internal layer of an intermetallic alloy hardsuperconductor comprising the steps of placing a tube made of a highermelting component of the alloy in a non-oxidizing atmosphere andannealing it, maintaining the lower melting point component of the alloyin a non-oxidizing atmosphere in molten state to degas it, pouring themolten metal into the tube while maintaining their temperatures belowthe reaction temperature at which their brittle intermetallic alloys areformed, cooling the tube, cold working the tube down to wire size toextend the interface between the core and sheath thereby making thesheath readily wettable by the core materials, cold rolling the wire toflatten it to ribbon form and to further reduce its cross sectionthickness after flattening the wire to ribbon form, and then heating theribbon at the reaction temperature to form the desired intermetallic,hard superconductor alloy as a diffusion layer inside the ribbon.

2. A method of making ductile superconductors providing high currentdensities through an internal layer of an intermetallic alloy hardsuperconductor comprising the steps of placing a tube made of a highermelting component of the alloy in a non-oxidizing atmosphere,maintaining the lower melting component of the alloy in said atmosphere,in molten state, pouring the molten metal into the tube whilemaintaining the temperatures of the tube and molten metal below thereaction temperature at which their brittle intermetallic alloys areformed, cooling the tube, then drawing the filled tube down to wiresize, to extend the interface between the core and sheath thereby makingthe sheath readily wettable by the core materials, then flattening thetube to ribbon, and then heating the ribbon at the reaction temperatureto form the desired intermetallic alloy as diffusion layer at the innersurface of the sheath.

3. The method of claim 2 wherein the higher melting component is niobiumand the lower melting component is selected from the class consisting oftin, gallium, thallium, silicon, aluminum, germanium, indium, andmixtures thereof.

4. The method of claim 3 wherein the higher melting component is niobiumand the lower melting component is tin and the reaction temperature ismaintained between 900 and 1200 C. a

5. The method of claim 4 wherein the ribbon is cold rolled to reduce itscross section thickness to less than .002 inch before heat treating atthe reaction temperature.

6 6. The method of claim 2 wherein the higher melting 2,887,763 5/1959Snavely 29-1555 component is selected from the class consisting ofniobium, 3,060,557 10/1962 Rostoker et a]. 29-194 tantalum and vanadium,and mixtures thereof. 3,057,048 10/1963 Hirakis 29-194 3,181,936 5/1965Denny et a1. 7 References Cited by the Examiner 5 UNITED STATES PATENTSWHITMORE A. WILTZ, Primary Examiner. 2,877,539 3/1959 Kinnan 55 5 P. M.COHEN, Assistant Examiner.

1. A METHOD OF MAKING DUCTILE SUPERCONDUCTORS PROVIDING HIGH CURRENTDENSITIES THROUGH AN INTERNAL LAYER OF AN INTERMETALLIC ALLOY HARDSUPERCONDUCTOR COMPRISING THE STEPS OF PLACING A TUBE MADE OF A HIGHERMELTING COMPONENT OF THE ALLOY IN A NON-OXIDIZING ATMOSPHERE ANDANNEALING IT, MAINTAINING THE LOWER MELTING POINT COMPONENT OF THE ALLOYIN A NON-OXIDIZING ATMOSPHERE IN MOLTEN STATE TO DEGAS IT, POURING THEMOLTEN METAL INTO THE TUBE WHILE MAINTAINING THEIR TEMPERATURES BELOWTHE REACTION TEMPERATURE AT WHICH THEIR BRITTLE INTERMETALLIC ALLOYS AREFORMED, COOLING THE TUBE, COLD WORKING THE TUBE DOWN TO WIRE SIZE TOEXTEND THE INTERFACE BETWEEN THE CORE AND SHEATH THEREBY MAKING THESHEATH READILY WETTABLE BY THE CORE MATERIALS, COLD ROLLING THEWIRE TOFLATTEN IT TO RIBBON FORM AND TO FURTHER REDUCE ITS CROSS SECTIONTHICKNESS AFTER FLATTENING THE WIRE TO RIBBON FORM, AND THEN HEATING THERIBBON AT THE REACTION TEMPERATURE TO FROM THE DESIRED INTERMETALLIC,HARD SUPERCONDUCTOR ALLOY AS A DIFFUSION LAYER INSIDE THE RIBBON.